System and method for tunable patterning and assembly of particles via acoustophoresis

ABSTRACT

A method of ordering particles suspended in a solution includes providing anisotropic particles suspended in a solution to a channel, wherein the anisotropic particles are unordered when entering the channel. The method further includes applying sound waves to the channel, wherein the frequency of the sound wave is tuned to create one or more columns of particles oriented in the same direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.62/141,053, filed on Mar. 31, 2015, and entitled “SYSTEM AND METHOD FORTUNABLE PATTERNING AND ASSEMBLY OF PARTICLES VIA ACOUSTOPHORESIS,” thedisclosure of which is incorporated by reference in its entirety.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with Government support under Grant (orContract) No. W911NF-09-D-0001, awarded by the Army Research Office. TheGovernment has certain rights in this invention.

TECHNICAL FIELD

The present disclosure is related generally to particle organization,and in particular to tunable patterning of particles viaacoustophoresis.

BACKGROUND

The manipulation of particles suspended in a solution is utilized in anumber of applications, including filtration, biochemical diagnostics,and materials processing. Manipulation of these particles is typicallyaccomplished via chemical, magnetic, electric, or rheological techniquesthat require either functionalized particles or a narrow range ofparticle/fluid properties.

It would be beneficial to develop a system and method of organizing andpatterning particles suspended in a solution that allows particleorientation and patterning particles suspended in a solution that allowsparticle orientation and location to be controlled.

BRIEF SUMMARY OF THE INVENTION

An embodiment provides a method of ordering particles suspended in asolution in a channel, wherein the particles are unordered when enteringthe channel. The method further includes applying sound waves to thechannel, wherein the frequency of the sound wave is tuned to create oneor more columns of particles oriented in the same direction.

Another embodiment provides an apparatus for delivering orderedparticles to a printhead of a three-dimensional printer. The apparatusincludes an acoustophoresis device, a piezoelectric device, and aprinthead. The acoustophoresis device includes at least a first inlet,at least a first outlet, and a channel defined between the first inletand the first outlet. The piezoelectric device is adhered to a surfaceof the acoustophoresis device to generate sound waves within at least aportion of the channel. The solution provided to the first inlet of theacoustophoresis device includes particles, wherein the acoustic wavesgenerated by the piezoelectric device organizes the particles into oneor more columns along an axis parallel to a direction of flow of thesolution. Then at least one column of particles is aligned with thefirst outlet, and wherein the first outlet delivers the one or morecolumns of particles to the printhead.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an acoustophoresis device that utilizesacoustic waves to organize particles suspended in a fluid into one ormore columns according to an embodiment of the present invention.

FIG. 2 is a magnified view that illustrates the organization ofanisotropic particles within a channel of the acoustophoresis deviceaccording to an embodiment of the present invention.

FIGS. 3a-3d are microscopic views of various shaped particles both inthe absence of and presence of acoustic waves according to an embodimentof the present invention.

FIGS. 4a-4c are cross-sectional views of acoustophoresis devicesutilized to deliver anisotropic particles organized into a columnaccording to an exemplary application of the present invention.

FIGS. 5a-5c are microscopic views and graphs illustrating therelationship between columnar spacing and number of columns according toembodiments of the present invention.

FIGS. 6a-6e illustrates stacking of anisotropic particles in the x-zplane in the presence of acoustic waves and maintenance of anisotropicparticle organization following removal of the acoustic waves accordingto an embodiment of the present invention.

FIGS. 7a-7d illustrate the relationship between particle shape/size,flow rate, and separation efficiency according to embodiments of thepresent invention.

FIGS. 8a-8c illustrate the relationship between voltage applied to thepiezoelectric device and formation of one or more columns according toan embodiment of the present invention.

DETAILED DESCRIPTION

The present invention provides a system and method of organizingparticles using acoustic waves. In particular, the acoustic wavesprovide a primary focusing force that drives particles to the pressurenode of the standing wave, and secondary scattering forces that interactwith the particles to align the particles end-to-end with one anotherwithin a given column, and to repel particles aligned with separatecolumns. The combination of these competing forces allows particles tobe organized into a plurality of parallel columns. By increasing theprimary focusing force, the plurality of columns can be collapsed into ahighly ordered column or array of particles. In this way, the presentinvention provides a system and method of selectively organizingparticles in a way that is desirable for a particular application viaselective modification/tuning of the acoustic waves applied. Benefits ofutilizing acoustic waves to organize the orientation and location ofparticles include the ability to organize particles of various sizes andof different materials (i.e., material agnostic).

FIG. 1 is a perspective view of acoustophoresis system 10 that utilizesacoustic waves to organize particles suspended in a fluid into one ormore columns according to an embodiment of the present invention. System10 includes reservoir 12, acoustophoresis device/chip 14, andpiezoelectric device 16. Reservoir 12 holds a colloidal solution thatincludes microscopically dispersed insoluble particles. A variety ofparticle sizes may be utilized for particles with at least one dimensionon the order of 100 nm or larger. Depending on the size, density, and/orbuoyancy of the particles in the solution, sonicator 17 may be utilizedto maintain a desired distribution of particles within the solution. Forexample, in one embodiment sonicator 17 is operated at a frequency ofapproximately 20 kHz to maintain the desired distribution of particles.In other embodiments, sonicator 17 may not be required to maintain thedesired distribution of particles within the colloidal solution, and apassive reservoir may be utilized in isolation.

In the embodiment shown in FIG. 1, acoustophoresis device 14 ismanufactured as a semiconductor device that includes glass layer 18 andsemiconductor substrate 20. Acoustophoresis device 14 includes inlet 22,channel 24, and outlets 26 and 28. A variety of fabrication techniquesmay be utilized to fabricate acoustophoresis device 14, although inother embodiments the device may be fabricated using traditional, ratherthan semiconductor fabrication techniques. In the embodiment shown inFIG. 1, semiconductor substrate 20 is a silicon substrate having athickness of approximately 525 μm. The silicon substrate is coated withan aluminum oxide film via an atomic layer deposition process to serveas a hard etch mask, which is patterned using photolithographictechniques and etched using a buffered oxide etch. Channel 24 is etchedinto semiconductor substrate 20 using a deep-reactive ion etchingtechnique to provide a channel depth of approximately 150 μm. Thealuminum oxide film (e.g., hard etch mask) is subsequently removed via abuffered oxide etch (e.g., buffered HF or BOE process) and the processedsubstrate is anodically bonded to glass layer 18 having pre-drilledfluidic vias to serve as inlet 22 and outlets 26 and 28.

Piezoelectric device 16 is bonded to the surface of acoustophoresisdevice 14 in the vicinity of channel 24. An excitation signal providedto piezoelectric device 16 generates acoustic waves directed towardchannel 24. The magnitude and frequency of the acoustic waves arerelated to the magnitude and frequency of the excitation signal providedto piezoelectric device 16, which may be selectively modified to achievethe desired result. As described in more detail below, modifying theamplitude and frequency of the acoustic waves allows the particles to beorganized in a controlled and/or tunable manner.

In the embodiment shown in FIG. 1, the solution retained withinreservoir 12 is supplied to acoustophoresis device 14 via inlet 22. Asdescribed in more detail below, the rate at which solution is providedto inlet 22 determines the flow rate through acoustophoresis device 14.Typically, organizing particles via traditional methods requiresrelatively low flow rates. A benefit of the present invention, asdescribed below, is the ability to organize particles in the presence offlow. Upon entering channel 24 from inlet 22, particles are evenlysuspended and randomly organized within the solution. Application ofacoustic waves from piezoelectric device 16 causes the particles toorganize within channel 24. In the embodiment shown in FIG. 1, theportion of channel 24 extending on the outlet side of piezoelectricdevice 16 is bifurcated such that particles located in the center ofchannel 24 are directed to outlet 26 and particles located on the edgesof channel 24 are directed to second or waste outlet 28. In otherembodiments, channel 24 may be designed with a different number ofoutlets and/or geometries based on the particulars of the application.

FIG. 2 is a magnified view that illustrates the organization ofanisotropic particles within channel 24 of the acoustophoresis device 14according to an embodiment of the present invention. In the embodimentshown in FIG. 2, the particles are rod-like in shape, and are thereforeanisotropic particles that exhibit attributes based on the orientationof the particles (e.g., greater strength in one direction than another).Upon entering channel 24, the anisotropic particles are distributedrandomly throughout the channel and are oriented in random directions.The anisotropic particles flow in the z direction toward outlets 26 and28. As the particles move past piezoelectric device 16, the acousticwaves generated by piezoelectric device 16 cause the particles toorganize. As discussed in more detail below, two competing forces act onthe particles to organize and pattern the particles. The first is aprimary focusing force that drives particles that are denser (e.g.,anisotropic particles in this example) to the pressure node of thestanding acoustic wave. The second is a scattering force, which isattractive when the interacting particles are aligned parallel to theacoustic wave and repulsive when the particles are aligned perpendicularto the acoustic wave. As a result of the attractive scattering forces,anisotropic particles are snapped end-to-end in a given column (i.e.,along the z axis). In addition, as illustrated in other embodiments,repulsive forces act to keep multiple assembled columns from collapsinginto a single column.

In the embodiment shown in FIG. 2, however, anisotropic particles arealigned end-to-end in a single column near the middle of channel 24(i.e., along the x axis). That is, the acoustic standing wave generatedby piezoelectric device 16 has a wavelength approximately twice that ofthe channel width, and therefore has a pressure node near the center ofchannel 24. Once organized, the particles tend to retain thisorganization even after leaving the region of the channel located underpiezoelectric device 16. As a result, particles organized into a columnas shown in FIG. 2 would be provided to outlet 26 (shown in FIG. 1). Abenefit of this arrangement is that outlet 26 is provided with a muchhigher volume fraction of particles than that provided at inlet 22. Inaddition, the anisotropic particles shown in FIG. 2 are oriented suchthat anisotropic attributes of the particles can be utilized. This is animportant consideration in fields such as 3D printing, in which thestrength of the printed device (i.e., the finished product) depends inpart on the orientation of the particles deposited. One of the benefitsof the embodiment illustrated in FIGS. 1 and 2 is the ability to orderthe particles in the presence of flow (i.e., while the particles areflowing through the channel from inlet to outlet). As described in moredetail below, the dimensions of channels 24 may also be modified toaccommodate different applications. In some embodiments, the height(i.e., y-axis) may be increased to allow for the stacking of columns inthey direction. In other embodiments, the width (i.e. x-axis) of channel24 may be modified to accommodate more or fewer columns stacked adjacentto one another.

FIGS. 3a-3d are microscopic views of various shaped particles both inthe absence of and presence of acoustic waves according to an embodimentof the present invention. In each figure, the left-hand side illustratesthe particles in the absence of acoustic waves, while the right-sideillustrates the particles in the presence of acoustic waves. FIG. 3aillustrates spherical particles (i.e., non-anisotropic) having adiameter of approximately 20 μm; FIG. 3b illustrates rod-like particles(i.e., anisotropic) having a length of approximately 50 μm and adiameter of approximately 10 μm; FIG. 3c illustrates bow-tie shapedparticles (i.e., anisotropic) having a height of approximately 80 μm, awidth of approximately 20 μm, and a length of approximately 40 μm; andFIG. 3d illustrates brick shaped particles (also anisotropic) having aheight of approximately 50 μm, a length of approximately 20 μm, and awidth of approximately 10 μm.

The embodiment shown in FIG. 3a illustrates how application of acousticwaves cases the spherical particles to group together within theapproximate center of the channel at the pressure node of the standingacoustic wave. Similarly, in the embodiment shown in FIG. 3b ,application of acoustic waves causes the cylindrical particles to alignwithin the approximate center of the channel. However, because thecylindrical shaped particles are anisotropic, scattering forces causethe particles to snap end-to-end, so that they are aligned along thelong axis of each cylindrical particle. Similarly, the bow-tie shapedparticles illustrated in FIG. 3c and the brick-shaped particles shown inFIG. 3d due to anisotropic attributes—are snapped end-to-end. Theembodiments shown in FIGS. 3b-3d therefore illustrate the ability notonly to control the location of the particles within the channel, butalso to control the orientation of those particles. This is particularlybeneficial in applications in which directional attributes of theparticles provide advantageous benefits, such as in 3D printingapplications, discussed below. It should be understood that thedimensions described with respect to FIGS. 3a-3d are exemplary, andother embodiments may utilize particles of different sizes and/orgeometries.

FIGS. 4a-4c are cross-sectional views of acoustophoresis devicesutilized to deliver anisotropic particles organized into a columnaccording to exemplary embodiments of the present invention. Each of theembodiments shown in FIGS. 4a-4c illustrate use of acoustophoresisdevices to deliver anisotropic particles to a 3D printhead, although itwill be recognized that in other embodiments the acoustophoresis devicesshown in FIGS. 4a-4c may be utilized in other applications.

FIG. 4a illustrates a single-stage acoustophoresis device, FIG. 4billustrates a two-stage acoustophoresis device, and FIG. 4c illustratesa two-stage acoustophoresis device that further includes a UV curingwindow

In the embodiment shown in FIG. 4a , single stage acoustophoresis device30 includes three-dimensional printhead 31, which is used to depositmaterial onto substrate 32 layer by layer, allowing a three-dimensionalobject to be constructed. An important attribute to 3D printing is thevolume fraction of particles (i.e., concentration) that is delivered tothe printhead. A common problem with particle delivery to printhead 31is clogging of the printhead. As described below, the ability to locateparticles within the center of the channel and orient the particlesalong their long axis allows a high volume fraction of particles to bedelivered to the printhead. A benefit of focusing particles in anordered manner is that it allows for particle concentrations thatotherwise would result in clogging of the printhead.

In the embodiment shown in FIG. 4a , acoustophoresis device 30 includeschannel 34, piezoelectric device 36, outlet 38, and waste outlet 40. Acolloidal solution that includes suspended, anisotropic particles isflowed into channel 34. Acoustic waves generated by piezoelectric device36 force the denser particles to the center of channel 34, to thepressure node of the standing acoustic wave. Scattering forces act onthe anisotropic particles to snap them end-to-end, resulting in thecylinder shaped particles aligning into a single column in the middle ofchannel 34. As a result, anisotropic particles forced to the center ofchannel 34 are flowed into outlet 38, which provides the alignedparticles to printhead 31. The received particles are deposited byprinthead 31 onto substrate 32.

The embodiment shown in FIG. 4a takes advantage of the ability tocontrol the location and orientation of the particles for purpose ofincreasing the density of particles directed to printhead 31. As aresult, rather than providing randomly distributed and orientedparticles to printhead 31, a single column of particles—each snappedend-to-end—is presented to printhead 31 via outlet 38. When theorientation of the particles is not controlled, the randomly orientedparticles have the effect of clogging printhead 31. Controlling theorientation of the particles allows a higher concentration of particlesto be provided to printhead 31, while preventing clogging of theprinthead. Excess solution is routed via waste outlets 40 to a wastecollection/recycling system.

Although not shown in FIG. 4a , in some embodiments additional steps aretaken to ensure the particles—once ordered—remain that way when providedto printhead 31. This may be particularly important if the distancebetween piezoelectric device 36 and printhead 31 is substantial. Thismay include photopolymerization or thermal curing of thesolution/particles upon exiting the portion of the channel in which theacoustic waves are generated. Photopolymerization utilizes light (e.g.,ultra-violet (UV) light) to cure the colloidal solution (e.g., hydrogelor photocurable epoxy type solution) in which the particles aresuspended, thereby preserving the ordering of the particles. In oneembodiment, a light guide coupled to a LED is utilized to illuminate thecolloidal solution for a time period sufficient to provide the desiredphotocuring. In other embodiments, particle ordering may be promoted byadding macromolecules that promote particle cohesion. For example, inone embodiment described in more detail with respect to FIG. 6d utilizesnon-specific binding with DNA. This provides the desired cohesion, butstill provides sufficient flexibility to allow subsequent manipulationof the particle pattern in the solution. In still other embodiments,ordering is preserved by utilizing a fluid with sufficient viscosity andelasticity to retain the shape of the solution during deposition. Forexample, in one embodiment, an epoxy-type fluid may be utilized thatincludes fillers to obtain the desired viscosity and elasticity duringthe deposition process. In addition, embodiments utilizes an epoxy-typefluid may benefit from thermal curing, wherein the ordered anisotropicparticles suspended in the epoxy-type fluid are thermally cured prior todeposition to help maintain the ordering of the particles.

As described above, a benefit of providing ordered particles toprinthead 31 is that it provides a greater concentration of particles tothe printhead without causing clogging of the printhead. In addition,the ability to orient anisotropic particles being deposited on substrate32 allows the product being built to take advantage of the anisotropicbenefits of the particles. For example, the cylinder shaped particlesshown in FIG. 4 may exhibit greater strength in one direction thananother, and by depositing them in a desired orientation the productbeing constructed may take advantage of the directional attributes ofthe particles. An object constructed with all particles oriented in aparticular direction may therefore exhibit additional strength over thatprovided by the same particles randomly oriented.

In the embodiment shown in FIG. 4b , acoustophoresis device 42 is atwo-stage device—including first stage 44 and second stage 46. In theembodiment shown in acoustophoresis device 42 includes channel 48, firstpiezoelectric device 50, waste outlets 52, first outlet 54, secondpiezoelectric device 56, and second outlet 58. As described with respectto FIG. 4a , a colloidal solution that includes suspended, anisotropicparticles is flowed into channel 48. In first stage 44, acoustic wavesgenerated by piezoelectric device 50 force the denser particles to thecenter of channel 48, to the pressure node of the standing acousticwave. Scattering forces act on the anisotropic particles to snap themend-to-end, resulting in the cylinder shaped particles aligning into oneor more columns (e.g., two columns in the example illustrated in FIG.4B). As a result, anisotropic particles forced to the center of channel48 are flowed into first outlet 54. Rather than be provided directly toa printhead, particles flowed to first outlet 54 are provided to secondstage 46 for additional organizing, as discussed below. Excess solutionis routed via waste outlets 52 to a waste collection/recycling system(not shown).

In addition, the embodiment shown in FIG. 4b includes a second stage 46,which includes second piezoelectric device 56. Anisotropic particlesprovided by first outlet 54 are further organized by secondpiezoelectric device 56. In one embodiment the organization of particlesprovided by first and second stages 44 and 46, respectively, may befine-tuned for different purposes. For example, in one embodiment firststage 44 is configured to provide filtering of particles for provisionto first outlet 54 (i.e., to provide a high concentration of particles),while second stage 46 is configured to provide organization into adesired column formation (e.g., one-column, two columns, etc.) forprovision to printhead 58. As shown in FIG. 4b , anisotropic particlesorganized in second stage 46 are provided to printhead 58 for depositiononto substrate 60.

In the embodiment shown in FIG. 4c , acoustophoresis device 62 may bedescribed again as a two-stage device—including first stage 64 andsecond stage 66. However, the embodiment shown in FIG. 4c furtherincludes a UV curing of the particles prior to deposition.

In the embodiment shown in FIG. 4c , acoustophoresis device 62 includeschannel 68, first piezoelectric device 70, waste outlets 72, firstoutlet 74, second piezoelectric device 76, and second outlet 78. Asdescribed with respect to FIG. 4a , a colloidal solution that includessuspended, anisotropic particles is flowed into channel 68. In firststage 64, acoustic waves generated by piezoelectric device 70 force thedenser particles to the center of channel 68, to the pressure node ofthe standing acoustic wave. Scattering forces act on the anisotropicparticles to snap them end-to-end, resulting in the cylinder shapedparticles aligning into one or more columns (e.g., two columns in theexample illustrated in FIG. 4c ). As a result, anisotropic particlesforced to the center of channel 68 are flowed into first outlet 74.Rather than be provided directly to a printhead, particles flowed tofirst outlet 74 are provided to second stage 66 for additionalorganizing, as discussed below. Excess solution is routed via wasteoutlets 72 to a waste collection/recycling system (not shown).

Anisotropic particles provided by first outlet 74 are further organizedby second piezoelectric device 76. Once again, the organization ofparticles provided by first and second stages 64 and 66, respectively,may be fine-tuned for different purposes. Anisotropic particlesorganized by second piezoelectric device 76 are provided to secondoutlet 78. In the embodiment shown in FIG. 4c , anisotropic particlesprovided to outlet 78 are combined with lubricating fluid provided byinput channels 79. The combination of anisotropic particles andlubricating fluid are provided to ultra-violet (UV) curing window 82,wherein UV light is provided to cure the anisotropic particles in theirpatterned or organized state. Once cured, the anisotropic particlesremain aligned within the patterned configuration, and can be providedto printed head 86 for deposition onto substrate 88. In otherembodiments, rather than photocuring via curing window 82, the orderedanisotropic particles may be thermally cured after being deposited ontosubstrate 88. As discussed above, epoxy-type fluids may be utilizedwhich can be thermally cured to retain the shape/ordering of theanisotropic particles.

A benefit of the 3D or additive techniques described with respect toFIGS. 4a-4c is the ability to implement net-shape manufacturingtechniques in which the shape of the printed product is very close tothe final shape of the product. That is, the embodiments shown in FIGS.4a-4c does not require subsequent cutting or molding of the orderedparticles. In addition, ordering of the particles allows the particlesto be deposited with a desired orientation to provide additionalstrength/resiliency attributes to the finished product. FIG. 5a is amicroscopic view of anistropic particles organized into two or morecolumns according to exemplary embodiments of the present invention.Microscopic view 100 a illustrates organization of the particles intotwo columns; microscopic view 100 b illustrates organization of theparticles into three columns, microscopic view 100 c illustratesorganization of the particles into four columns, microscopic view 100 dillustrates organization of the particles into five columns, microscopicview 100 e illustrates organization of the particles into six columns,and microscopic view 100 f illustrates organization of the particlesinto seven columns.

The embodiments shown in FIG. 5a illustrate the ability to effectivelytune the organization of particles into multiple columns. Theorganization of particles in this manner is a result of competitionbetween the primary focusing force and the secondary scattering forces.Scattering forces are attractive when the interacting particles arealigned parallel to the acoustic wave and repulsive when the particlesare aligned perpendicular to the acoustic wave. As a result of theattractive scattering forces, anisotropic particles are snappedend-to-end in a given column (i.e., along the z axis). In contrast, therepulsive forces act to keep multiple assembled columns from collapsinginto a single column. For example, microscopic view 100 a illustratestwo columns of aligned anisotropic particles. Primary forces act on theparticles to push them toward the pressure node of the standing acousticwave (in this embodiment, to a point located between the two columns).However, repulsive scattering forces, balancing the primary force, actto maintain the columns a distance d from one another. As discussed inmore detail below, increasing the magnitude (i.e., power) of theacoustic wave increases the magnitude of the primary force, which atsome point overcomes the magnitude of the secondary scattering force andcauses the two columns to collapse into a single column.

The ability to tune the organization of particles into two or morecolumns is illustrated in microscopic views 100 a-100 f. Throughcontrolling the particle concentration, the anisotropic particles can beselectively ordered into a plurality of columns (seven in the embodimentshown in microscopic view 100 t). Increasing the magnitude of theacoustic wave has the effect of collapsing the columns into adjacentcolumns. In addition, as illustrated in FIGS. 5a-5c , the distancebetween adjacent columns is related to the total number of columns. Asthe number of columns increases, the distance between each of thecolumns decreases.

This concept is illustrated in schematic fashion by FIG. 5b , whichillustrates how the changing distance between columns is a function ofnumber of columns according to an embodiment of the present invention.The bottom axis of the graph shown in FIG. 5b is defined as the positionalong the channel divided by the width of the channel. Although thisaxis would extend from −1 to 1, only −0.1 to 0.1 is shown, as columnsare created within this region. As discussed above with respect to FIG.5a , as the number of columns increases (toward the top of the graph),the distance between the columns decreases. FIG. 5b also illustrates howthe columns are centered around the center of the channel, presumablythe location of the pressure node of the standing wave.

FIG. 5c is a chart that illustrates spacing between columns as afunction of both particle type and number of columns. The y-axisdescribes the distance d between columns, while the x-axis describes thetotal number of columns N_(c). The chart in FIG. 5c illustrates thedifference in spacing between anisotropic particles (cylindrical shaped)and non-anisotropic particles (spherical). In this case, particles werechosen with similar particle volumes so that the magnitude of theprimary acoustic radiation force is similar between different particleshapes, FIG. 5c once again illustrates that as the number of columnsincreases, the spacing between each of the columns decreases. Inaddition, the example shown in FIG. 5c illustrates that the spacingbetween columns of anisotropic particles are greater than the spacingbetween columns of non-anisotropic particles. For example, whenanisotropic particles are divided into two columns, the distance betweenthe columns is approximately 38 μm. When the non-anisotropic particlesare divided into two columns, the distance between the columns isapproximately 32 μm. As the number of columns increases, the distancebetween the columns decreases, but the spacing between anisotropicparticles remains greater than the spacing between non-anisotropicparticles. Thus, the embodiment shown in FIG. 5c illustrates the effectparticle size and shape has on spacing between columns.

FIGS. 6a-6e illustrate an additional aspect of organizing particles, inwhich rather than particles being configured in a single column, thecolumns are essentially stacked on top of one another in the y-direction(as shown in FIG. 2). In addition, FIGS. 6a-6e illustrate variousmethods of maintaining this stacked array configuration of particles viamicroscopic masonry according to various embodiments of the presentinvention.

The view shown in FIGS. 6a and 6b represents a top view of the channeltaken in the x-z plane (i.e., looking down into the channel). Inparticular, this embodiment shows that in the presence of acousticwaves, assuming a channel with sufficient height, a stacked array ofparticles can be formed with the stack extending out of the page in they direction. In the embodiment shown in FIG. 6a , a voltage ofapproximately 40 V_(pp) is applied to piezoelectric device, although inother embodiments the voltage may be increased/decreased as appropriatebased on the pattern desired and the particles being manipulated. Whenthe piezoelectric device is turned On, the particles are stacked into anarray near the center of the channel. When the piezoelectric device isturned Off, gravity begins to pull on the stacked array of particles asshown in FIG. 6b , as the stacked array turns sideways to expose how theparticles are stacked on top of one another in an array format. Thestacked array may also be described as a ribbon-like structure. Benefitsof the configuration of particles shown in FIG. 6b is the sharedorientation of the particles as well as the close-packing of theparticles in a micro-masonry configuration. Benefits of thisconfiguration include added strength associated with the sharedorientation and dense packing of the particles. In the embodiment shownin FIGS. 6a-6e , the particles are anisotropic particles that arewell-suited for micro-masonry configurations.

In this way, FIGS. 6a and 6b illustrate another way in which anisotropicparticles may be organized. Rather than being organized into one or moreplurality of columns, the particles are organized into a stacked arrayof particles, forming a ribbon-like structure. Although theparticles—once patterned—tend to retain their shape for at least anominal amount of time, in some embodiments it may be required orbeneficial to retain the desired pattern. In the embodiment shown inFIG. 6c , a photopolymerization of the hydrogel solution in which theparticles are suspended is utilized to preserve the desired pattern. Asdiscussed previously, photopolymerization utilizes a hydrogel solutionthat can be photocured using application of light to the solutionsurrounding the ordered particles. In 3D printing applications, therates of polymerization must be balanced with the desired transportproperties of the suspension being provided to the print head. In otherembodiments, rather than photopolymerization, ordering is preserved byutilizing a fluid with sufficient viscosity and elasticity to retain theshape of the solution during deposition. For example, in one embodiment,an epoxy-type fluid (rather than a hydrogel) may be utilized thatincludes fillers to obtain the desired viscosity and elasticity duringthe deposition process. In addition, the epoxy-type fluid may bethermally cured in order to further retain the ordering of anisotropicparticles.

In the embodiment shown in FIG. 6d , pattern stability is promoted byadding macromolecules that promote particle cohesion according toanother embodiment of the present invention. In one embodiment, thearray or pattern of particles is held together via non-specific bindingwith DNA. For example, in the embodiment shown in FIG. 6d (including themagnified portion), the stacked array of rod-shaped particles (in thiscase, glass rods) are focused in a chaotropic solution with dispersed.DNA. The chaotrope drives DNA onto the surfaces of the rod-shapedparticles, creating cohesive interactions between a single DNA strandand adjacent rods in packed arrays. In one embodiment the DNAconcentration utilized is sufficient to coat the surfaces of theparticles as well as the surface of the channel walls.

In the embodiment shown in FIG. 6e , a sequence of still shots is takenillustrating how by sweeping the frequency to set up acoustic standingwaves alternating from the x direction to the z direction can be used tofold the ribbon-like structure (shown in frames 102 a-102 i) multipletimes onto itself, resulting in an ordered three-dimensional array. Theembodiment illustrated in FIG. 6e utilizes DNA dispersed within thesolution to promote particle adhesion as described with respect to FIG.6 d.

In first frame 102 a the ribbon-like array of stacked particles arealigned in a single direction (i.e., parallel to the z-axis). In frames102 b-102 d, the direction of the acoustic wave generated by thepiezoelectric device is modified such that particles—as they exist inthe ribbon like structure—are driven to a position parallel with thex-axis as shown in frame 102 d. In this way, the ribbon-like structureis folded over on itself. In frames 102 e-102 i the direction of theacoustic waves generated by the piezoelectric device is returned to theoriginal orientation such that the ribbon-like structure is again foldedonto itself and the particles are once again aligned parallel to thez-direction. As a result of multiple folding operations, the initiallytwo-dimensional ribbon (such as that shown in FIG. 6a ) is folded ontoitself a number of times, thereby creating a three-dimensional shape.

FIGS. 7a-7d illustrate separation/filtering results for anisotropic andnon-anisotropic particles of various sizes by an acoustophoresis deviceaccording to embodiments of the present invention. FIGS. 7a and 7billustrate separation efficiencies for sphere-shaped particles (i.e.,non-anisotropic) of different concentrations in suspension—0.1 and 0.5nominal wt %, respectively—at excitation voltages of 5 V_(pp), 10V_(pp), and 15 V_(pp), and at flow rates of 10, 50, and 150 μL/min.Likewise, FIGS. 7c and 7d illustrate separation efficiencies formicro-rod shaped particles (i.e., anisotropic particles) of differentsizes—0.1 and 0.5 nominal wt %, respectively—at excitation voltages of 5V_(pp), 10 V_(pp), and 15 V_(pp), and at flow rates of 10, 50, 150, 300and 600 μL/min. in each figure, separation of the particles at anexcitation voltage of 5 V_(pp) is illustrated by a circle, 10 V_(pp) isillustrated by a square, and 15 V_(pp) is illustrated by a diamond. Flowrates are denoted along the x-axis, while separation efficiencies aredenoted along the y-axis. Particle sizes were selected by volume to beapproximately equal in order to ascertain the effect of particle shapeon the separation efficiency achieved at each voltage.

FIGS. 7a and 7b illustrate separation efficiencies for micro-spheres. Inparticular, at low flow rates (i.e., 10 μL/min), the separationefficiency is approximately the same regardless of the excitationvoltage applied. In general, as the flow rate increases the separationefficiency of the micro-spheres decreases. In addition, at higher flowrates the excitation voltage applied has a greater effect on theseparation efficiency achieved.

FIGS. 7c and 7d illustrate separation efficiencies for micro-rods. Incontrast with the microspheres, the micro-rods illustrate increasedseparation efficiency at increased flow rates (e.g., as the flow rateincreases from 10 μL/min to 300 μL/min). Only at the very highest flowrates, when the flow rate approaches 600 μL/min does the separationefficiency of the micro-rods begin to decrease. This indicates that theanisotropic effect of the micro-rods improves separation efficiency inthe presence of flow.

FIGS. 8a-8c illustrate the relationship between voltage applied to thepiezoelectric device and formation of one or more columns according toan embodiment of the present invention. In FIG. 8a , an excitationvoltage of 20 V_(pp) is applied to piezoelectric device. The resultingbalance of primary forces and secondary scattering forces result in theparticles being organized into three parallel columns. That is, at thisexcitation, the scattering force between the various columns balancesthe primary force attempting to collapse the columns to the center(i.e., to the pressure node of the acoustic wave). In FIG. 8b , theexcitation voltage is increased to 24 V_(pp). The increase in excitationvoltage results in an increase in the magnitude of the acoustic waveprovided to the channel, and as a result an increase in the primaryforces applied to the particles. In response, two of the columns arecollapsed into a single column, resulting in a total of two columns.That is, at an excitation voltage of 24 V_(pp), primary forces andsecondary scattering forces are balanced with two columns. Similarly,when the excitation voltage is increased from 24 V_(pp) to 41 V_(pp),the primary force once again outweighs the secondary scattering forcesand causes the two columns shown in FIG. 8b to collapse into a singlecolumn.

While the invention has been described with reference to an exemplaryembodiment(s), it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

1. A method of ordering particles suspended in a solution, the methodcomprising: providing anisotropic particles suspended in a solution to achannel, wherein the anisotropic particles are unordered when enteringthe channel; and applying sound waves to the channel, wherein thefrequency of the sound wave is tuned to create one or more columns ofanisotropic particles oriented in the same direction.
 2. The method ofclaim 1, wherein an output of the channel is bifurcated such thatanisotropic particles organized into one or more columns of anisotropicparticles located in a center of the channel are directed to a firstoutlet and particles located outside of the center of the channel aredirected to a second outlet.
 3. The method of claim 1, wherein the soundwaves applied to the channel force the anisotropic particles to apressure node of the sound wave, and wherein scattering forces act onthe anisotropic particles to align them end-to-end.
 4. The method ofclaim 1, further including: photocuring the one or more columns ofanisotropic particles to preserve the ordering and orientation of theone or more columns of anisotropic particles, wherein the solution is ahydrogel solution.
 5. The method of claim 1, further including:thermally curing the one or more columns of anisotropic particles topreserve the ordering and orientation of the one or more columns ofanisotropic particles, wherein the solution is an epoxy-type solution.6. The method of claim 1, wherein the solution is a chaotropic solutiondispersed with macromolecules that bind with the one or more columns ofanisotropic particles to promote particle cohesion to preserve theordering and orientation of the one or more columns of anisotropicparticles.
 7. The method of claim 1, wherein applying sound waves to thechannel includes applying sound waves in a first stage to extract a highconcentration of anisotropic particles from the solution, and applyingsound waves in a second stage to order the extracted anisotropicparticles into a desired configuration.
 8. The method of claim 1,further including depositing the one or more columns of orientedanisotropic particles via a printhead.
 9. An apparatus for deliveringordered particles to a printhead of a three-dimensional printer, theapparatus comprising: a acoustophoresis device having at least a firstinlet, at least a first outlet, and a channel defined between the firstinlet and the first outlet; and a first piezoelectric device adhered toa surface of the acoustophoresis device to generate sound waves withinat least a first portion of the channel, wherein a solution comprisinganisotropic particles is delivered to the at least first inlet andwherein the anisotropic particles are organized into one or more columnsalong an axis parallel to a direction of flow of the solution bystanding acoustic waves generated by the piezoelectric device, whereinthe at least one column of anisotropic particles is aligned with thefirst outlet, and wherein the first outlet is in fluid communicationwith the printhead to deliver one or more columns of ordered,anisotropic particles to the printhead.
 10. The apparatus of claim 9,wherein the acoustophoresis device further includes a waste outlet,wherein anisotropic particles organized into one or more columns withina center of the channel are provided to the first outlet and whereinexcess solution is provided to the waste outlet.
 11. The apparatus ofclaim 9, wherein the acoustophoresis device includes a first stage and asecond stage, wherein the first stage includes the at least first inletand the first outlet, and wherein the second stage includes at least asecond inlet and a second outlet, wherein the at least second inlet isin fluid communication with the first outlet of the first stage andwherein the second outlet is in fluid communication with the printhead.12. The apparatus of claim 11, further including a second piezoelectricdevice adhered to a surface of the acoustophoresis device to generatesound waves within the second stage of the acoustophoresis device,wherein the first piezoelectric device generates sound waves within thefirst stage of the acoustophoresis device.
 13. The apparatus of claim 9,wherein the solution in which the anisotropic particles are suspended isan epoxy-type solution, and wherein the apparatus further includes athermal curing element located upstream of the printhead, wherein thethermal curing element thermally cures the epoxy-type solution topreserve the organization of the anisotropic particles.
 14. Theapparatus of claim 9, wherein the solution in which the anisotropicparticles are suspended is a hydrogel solution, and wherein theapparatus further includes a photo-curing window located upstream of theprinthead, wherein the photo-curing window allows incident light to curethe hydrogel solution to preserve the organization of the anisotropicparticles.
 15. A method of providing ordered particles to a printhead ofa three-dimensional printer, the method comprising: providinganisotropic particles suspended in a solution to an inlet of anacoustophoresis device, wherein the acoustophoresis device includes theat least one inlet, a primary outlet, a waste outlet, and a channeldisposed between the at least one inlet and the primary outlet; applyingsound waves to the channel of the acoustopheresis device to order theanisotropic particles in one or more columns along an axis parallel to aflow of solution through the channel, wherein the anisotropic particlesare oriented in the same direction as one another within the channel;receiving at the outlet one or more ordered columns of anisotropicparticles; and providing the one or more ordered columns of anisotropicparticles received at the outlet to the printhead for deposition
 16. Themethod of claim 15, wherein the acoustophoresis device further includesa first stage in fluid communication with the at least one inlet, anintermediate outlet, and one or more waste outlets, wherein sound wavesapplied to the first stage order the anisotropic particles in one ormore columns, wherein the ordered anisotropic particles are provided tothe intermediate outlet while excess solution is provided to the wasteoutlets.
 17. The method of claim 16, wherein the acoustophoresis devicefurther includes a second stage in fluid communication with theintermediate outlet, wherein sound waves applied to the second stageorder the anisotropic particles in one or more columns, wherein theordered anisotropic particles are provided to the primary outlet. 18.The method of claim 17, wherein sound waves applied to the first stageare generated by a first piezoelectric device and second waves appliedto the second stage are generated by a second piezoelectric device. 19.The method of claim 15, further including: photocuring the one or morecolumns of anisotropic particles to preserve the ordering andorientation of the one or more columns of anisotropic particles, whereinthe solution is a hydrogel solution.
 20. The method of claim 15, furtherincluding: thermally curing the one or more columns of anisotropicparticles to preserve the ordering and orientation of the one or morecolumns of anisotropic particles, wherein the solution is an epoxy-typesolution.